This invention relates in general to the technology of piezoelectricity as evinced in the applications of crystals, particularly quartz crystals, as resonators. In the technical and scientific applications of quartz crystal oscillator devices great emphasis is placed upon, and great benefits derived from, a reliably precise relationship between frequency and temperature, usually referred to as the frequency-temperature (f vs. T) characteristic of these devices. It is known in the art that a basic crystal oscillator, called an XO for crystal oscillator, can have the precision of its frequency-temperature characteristic enhanced by temperature compensation, TXCO, or by oven control, OCXO.
In a temperature-compensated crystal oscillator arrangement, an output signal from a temperature-sensitive transducer device, a thermistor for instance, is applied as a correction signal to a voltage-variable reactance element in the crystal resonator circuit. The consequent reactance changes compensate for deterministic variations in the crystal's frequency-temperature characteristic and make for about a twenty-fold improvment in the frequency-temperature variation accuracy over the same crystal as an XO.
OCXO's or oven-controlled crystal oscillators, comprehend disposing the crystal and any other temperature-sensitive components in a stable oven, the temperature of which is adjusted to the point where a curve of the crystal's frequency-temperature characteristic on Cartesian coordinates has zero slope. OCXO's provide about a thousand-fold improvement on the frequency-temperature variation accuracy over the same crystal as a PXO.
In the field of crystal oscillator devices then, a matter of great importance is accurately to detect and measure the temperature of the quartz resonator in order to be able to effect either measurement, e.g., as in thermometry, or compensation and/or control for the purpose of frequency stabilization against variations in temperature.
There have been a great many significant advances in the art in the attainment of temperature compensation and thermometric control of crystal oscillators and the present day temperature-compensated and/or oven-controlled quartz crystal oscillators are markedly improved over their earlier counterparts of a few years ago.
Improvements in temperature-compensation and oven-control techniques notwithstanding however, it has not always been recognized and understood that the precise dependence of frequency upon temperature and vice versa is not entirely free from small deviations related to the discrete crystal's past thermal history. The effect of hysteresis caused by a given thermal cycling history can become a troublesome characteristic in a precision resonator. In a quartz resonator which operates over a wide temperature range, for instance, these effects, called "retrace" or "hysteresis", are more pronounced due to the wide operational range. If, on the other hand the temperature range is decreased, the magnitude of hysteresis effect is decreased.
The patent application of Schodowski, entitled "Dual Mode Quartz Sensing Device", Ser. No. 487,560, filed Apr. 20, 1983, in issue, Notice of Allowance dated July 31, 1989, relates to an apparatus and system for generating a pair of c-mode harmonic signals with a single vibrating quartz resonator, combining the harmonic outputs of the resonator to a subtractive or additive result according to a multiplier relationship between the higher and lower harmonics of the c-mode harmonic signals generated, and using the resultant frequency to measure according to a nearly linear proportional relationship between the resultant frequency and temperatures. This composite signal is, advantageously, highly sensitive to temperature changes, and Schodowski utilize this feature turned back into the system of his invention to attain an improved thermometric response over a preselected temperature range.
To the extent that my invention herein described and claimed is highly advantageous in enhancing the operation of the Schodowski device, the disclosure of patent application Ser. No. 487,560 is incorporated herein by reference.
Thus, with the extent of the knowledge available on resonator temperature measurement thermal hysteresis and the available techniques in the art for selfcompensating and for otherwise detecting and offsetting the deleterious effects of this phenomenon on the accuracy, presision, and reliability of the f vs. T relationship in quartz crystal resonators, and in the light of the basic fact that hysteresis effect is not a highly precise phenomenon, it can be appreciated that, prior to the time of my invention, there has been no reasonably completely satisfactory way for determining the thermal hysteresis characteristics of quartz crystal resonators.